This invention relates in general to weighing systems. More specifically, it relates to an apparatus and method for controlling the humidity within the scale.
Changes in the humidity of the air present within a scale can cause changes in the output of the scale which result in inaccurate measurements of weight, mass, or count. This is a particularly troublesome problem for scales which use a variable capacitance transducer to convert an applied force to-be-measured into an electrical output signal. Force transducers of this type are described in U.S. Pat. Nos. 4,485,085; 4,558,600; and 4,463,614, commonly assigned with this application. A weighing system utilizing such a transducer is described in U.S. Pat. Nos. 4,382,479 and 4,513,831, also commonly assigned with the present application. Humidity variations are a problem because they change the dielectric constant of the air in the variable capacitance gap, which in turn directly changes the output signal in a way that does not correspond to the applied force. Humidity is also a problem in that it can lead to condensation on the capacitor electrodes, and it can alter the performance of an inductance coil and other circuit elements present in the scale. While a straightforward solution might appear to be to simply seal the interior of the scale, this is not a practical solution. One problem is that a pan or some equivalent structure located exterior to the scale housing must be mechanically connected to the interior to transmit the force produced by the object being weighed as it rests on the pan to the transducer. Typically, a rigid shaft transmits the force and it enters the housing through a hole. If the shaft-to-housing clearance is sealed, this will produce a resistance to a longitudinal motion of the shaft which introduces an error in the output signal. Therefore, ambient air, with its variations in humidity, enters the interior cavity of the scale. One known solution is to hermetically seal the humidity-sensitive elements in a bellows, typically one made of rubber or metal. This solution does isolate components from certain atmospheric changes, but changes in the barometric pressure produce varying forces on the bellows which can result in output signal errors. Rubber bellows "boots" also have material-related problems such as a high hysteresis, creep, difficulty in achieving a reliable seal, and aging. Another known approach is to measure the humidity within the interior scale cavity independently, and use this measurement to produce electronically a compensating signal to offset the humidity changes. This approach has several significant drawbacks. First, it is difficult to measure only the relative humidity. Most units are influenced by temperature variations and air flow, and it does not respond well at very low and very high relative humidity condition. Second, there is the additional cost and reliability problems associated with this additional sensor and its related electronics.
Another approach has used a cooler positioned within the housing to condense the water vapor selectively on its condensing surfaces. This solution has a disadvantage in that it requires a relatively long period of time--hours, or in some cases, as much as a day--to achieve a steady state humidity. However, even this steady state is not a reliably controlled condition. Sudden or large variations in the ambient air humidity ca introduce unacceptably large variations in the humidity internal to the scale.
Still another known solution, disclosed in U.S. Pat. No. 4,649,759, also commonly assigned with the present application, is to use a second reference, or "dummy", capacitor mounted on or near the variable gap transducer. This second capacitor has a fixed gap so that by monitoring changes in its capacitance one can measure the atmospheric changes and compensate for them. While this arrangement works well under 0% to 80% R.H. (but not over 80% R.H.), it is restricted for various practical reasons to circuitry which produces a voltage or current signal output, but not a frequency signal output.
It is also significant to note that in the prior art, humidity control has often not been a significant problem in scale design because the element that converted the applied force into an output signal was usually a strain gauge, mechanical spring system, or some other element that is substantially insensitive to humidity (if the relative humidity is less than 80%). Also, with prior art scales, including those operating on the variable capacitance principle, it has usually been necessary to recalibrate the scale frequently. Some laboratory scales recommend recalibration every day, but all known high accuracy scales require recalibration at periodic intervals shorter than six months.
It is therefore a principal object of the present invention to provide a humidity control system for a scale that reliably controls the humidity in the scale housing at a generally constant value, despite much larger variations in the relative humidity of the surrounding ambient air, and without introducing side effects such as a frictional resistance to the motion of force responsive elements or a long waiting period before the humidity control system is operational, or sensitivity to atmosphere pressure change.
Another principal object is to provide a humidity control system with the foregoing advantages which is cost effective and compatible with transducers and signal processing circuitry of all types, including circuits which produce a frequency signal output.
Still another object is to provide a humidity control system with the foregoing advantages which will work even at condensation conditions.
A further object is to provide a humidity control system with the foregoing advantages that changes its steady state operating conditions very gradually (a long "time constant") and therefore does not require frequent recalibration.
A still further advantage is to provide a humidity control system with the foregoing advantages which controls humidity for all internal components of the scale, and which can be readily retrofit to existing scales.